12 volt to mobo

Can I do away with the AC inverter and just plug into 12v from the car or do i need to worry about the starter cranking? do i need to but a capacitor inline in protect the mobo when the starter cranks?

A good "automotive" ATX supply might or should ride through cranking dips. A good battery should not dip below 9V else 8V and some 12V (auto) supplies work down to 8V & 6V.
Otherwise you need a 2nd battery to hold up the mobo.

A 2nd battery is popular anyhow as a means of separating PC power from the vehicle power for cranking dips etc as well as not flattening the main/cranking battery in case the PC is left on. It can also provide cleaner power for the PC (or audio) etc. (A battery is essentially like a BIG cap - only cheaper, and with far greater reserve time.)
AGM batteries are used (ie, sealed; hence legal for "internal" use) with 12V 7AH being a popular size unless larger is required.

An (automatic) battery isolator is essential so that the batteries are isolated when NOT being charged (ie, the engine not running hence alternator not charging).
But that can be as simple as a relay triggered by the alternator's charge-lamp circuit - search for "UIBI" & oldspark for some examples...
And don't forget protection for the battery interlink at EACH battery end. Protection means fuses, though I suggest self-resetting circuit breakers which are cheap (~$8) for circuits up to 50A. [You must dimension the relay and cable for the PC/mobo current PLUS battery recharge current. The latter is difficult to predict and IMO there isn't much worse than a blown fuse that you don't know about, hence the self-resetting circuit breakers in case a very flat battery causes a huge initial charge current thereby blowing the interlink fuse(s) and eventually losing the PC or fridge etc and killing their battery.]

A capacitor is not an efficient method of providing "gap" power. Batteries are cheaper, and they hold up for minutes or hours unlike the seconds for cas.

The PICO supplies I have seen are for "12V" meaning non-battery systems.
Automotive 12V (battery) systems are typically up to ~14.5V when charging; ~12.5V - 11.5V when not charging; and lower with heavier loads & cranking. But 16V is a typical "minimum" upper design voltage and IMO 8V is a reasonable "lower" design limit (ie, loads should handle 8-16V). But they also need transient/spike protection for higher voltages (eg, spikes of 100V-200V etc).